1. Field of the Invention
The invention relates to microwave, millimeter wave and submillimeter wave spectroscopy systems and components and in particular to an apparatus and method for generating and detecting terahertz signals using a single photoconductive element.
2. Description of the Related Art
Terahertz devices and systems generally employ electromagnetic energy between 300 gigahertz (300 GHz) and 3 terahertz (3 THz), or wavelengths from 100 to 1000 microns (0.1 to 1.0 millimeters), which is also referred to as the submillimeter or far-infrared region of the electronmagnetic spectrum.
One important application of terahertz systems is terahertz spectroscopy. Terahertz spectroscopy presents many new instrumentation and measurement applications since certain compounds and objects can be identified and characterized by a frequency-dependent absorption, dispersion, and/or reflection of terahertz signals which pass through or are reflected from the compound or object.
One way of generating terahertz radiation is by photomixing two optical signals of different frequencies using an optical-heterodyne converter or photomixer. Typical photomixer devices include low-temperature-grown (LTG) GaAs semiconductor devices, which have been used to generate coherent radiation at frequencies tip to 5 THz. The spectroscopy system typically uses two single frequency tuneable lasers, such as diode lasers, to generate two optical laser beams which are directed at the surface of the photomixer. By photoconductive mixing of the two beams in the semiconductor material, a terahertz difference frequency between the two optical laser frequencies is generated. In particular, a first laser generates radiation at a first frequency and a second laser generates radiation at a second frequency. The difference frequency, equal to the difference between the first and the second laser frequencies, is swept by the user from microwave through terahertz frequencies by changing the temperature of one or both lasers. Other types of tuning mechanisms exist, such as distributed-Bragg-reflector diode lasers with multiple electrodes, grating-loaded external cavities, etc.
Conventionally, a terahertz transceiver has a transmitter, including a first photomixer device, and a receiver, including a second photomixer device. The first photomixer device is optically coupled to the first and the second light source, and a first radiative element or antenna is electrically coupled to the first photomixer device. In operation, the first antenna radiates a terahertz signal, generated by the first photomixer device at the difference frequency, toward a sample material. Terahertz radiation transmitted through, or reflected from, the sample material is directed to the receiver and is incident on a second antenna, which is electrically coupled to the second photomixer device. The second photomixer device is also optically coupled to the first and second light sources. The second antenna generates a time varying voltage proportional to the terahertz return signal. Under illumination by the first and second light sources, the second photomixer generates a homodyne downconverted current signal in response to the time varying voltage generated by the second antenna. The downconverted signal is a measurement of the absorption or reflection by the sample material at each terahertz frequency. This is useful, for example, when used in conjunction with computer processing to identify unknown samples by comparing measured results to a library of reference spectra. This apparatus may also be used to characterize the frequency response characteristics of passive or active components and devices such as waveguides, filters, amplifiers, mixers, diodes, and the like designed to work at terahertz frequencies.
Typically, THz spectroscopy systems employ lock-in detection techniques to improve signal to noise levels. These lock-in techniques involve, in the transmitter, modulating the amplitude of the photomixer current in the first photomixer device by either chopping the optical signal or modulating a bias voltage applied to the first photomixer device. It is, however, important that the second photomixer device in the receiver does not pick up the modulation directly, but only via the THz signal received from the sample material. If the modulation is detected directly by the second photomixer device, the signal from the sample material will be masked. For this reason, separate photomixer devices are used in the transmitter and the receiver, and the separate photomixer devices are located away from each other.
US 2012/0326039 describes using optical phase modulation as an alternative to modulating the amplitude of the photomixer current in the first photomixer device. By converting the phase modulation into an amplitude modulation via an interference pattern, it is possible to maintain a constant bias across the photomixer device in the transmitter without requiring the use of a chopper. In this way, the level of the THz signal is increased.